The present invention relates generally to microstrip terminations, and to terminations used with optical modulators.
Terminations are common components in most microwave systems. Microstrip terminations are easy to manufacture using thin film technology, but the performance typically drops off rapidly with increasing frequency. Thin film technology typically uses an alumina substrate, with gold and resistor material sputtered onto it and then patterned with photolithography techniques to define microstrip transmission line traces and resistors. Thick films could also be used, but the thick film resistors do not function well at high frequencies (above 20 GHz).
FIG. 1 illustrates one standard microstrip termination, known as an edge ground circuit. In the microstrip 100 of FIG. 1, a microstrip transmission line 104, typically a metal line, is formed on the microstrip substrate 102, made of a dielectric such as alumina. An area of resistive material 106 is formed on the substrate 102 along the transmission line 104 near an edge ground. The edge ground is formed with a transmission line trace 110 connecting the resistive material 106 to the metal plated edge 108 which connects to a metal ground region 112 deposited on the bottom surface of the substrate. The resistive material 106 is used to terminate a signal propagating along the transmission line by matching the impedance of the transmission line and preventing reflection of the propagating signal.
FIG. 2 illustrates another microstrip termination typically used when grounding is desired away from a substrate edge. This termination 200 also includes a microstrip substrate 202 typically having a metal bottom layer 212, a transmission line 204, and an area of thin film resistive material 206. The substrate 202 also has a via 210 between the ground side of the resistor 206 and the bottom metal of the substrate 202. The substrate 202 often contains Monolithic Microwave Integrated Circuits (MMICs) connected to the transmission line 204 and the substrate 202 is often mounted on a carrier. A carrier is typically a thin metal plate, on the order of xc2xd to 1 mm thick, and provides the ground for the microstrip substrate and the MMICs thereon in addition to the metal bottom layer 212.
This termination of FIG. 2 further uses a ground via 210. The via 210 is formed from metal deposited in a hole in the substrate that extends from the area of metal 208 on the top surface of the substrate to the metal bottom layer 212. The termination shown in FIG. 2 can be placed anywhere in a subsystem circuit, but the performance is generally worse than the edge ground circuit of FIG. 1. The poor performance is due to the increased inductance to ground resulting from the small via.
A microstrip termination that provides acceptable performance at high frequencies over a wide bandwidth, but not at low frequencies or to DC, is the dot termination. Dot terminations are high return loss terminations capable of performing adequately at high frequency and over a wide bandwidth. Dot terminations typically do not require a ground. FIG. 3 shows a dot resistor 300 of the prior art. This dot resistor 300 typically includes a circular area of thin film resistive material 302. The circular area of resistive material 302 typically has a protruding region of resistive material, or tongue 304, which extends from the circular area and into contact with a metal trace 306 forming a transmission line. The thin film resistive material of the tongue 304 extends under the metal trace 306, assuring an overlap or connection between the metal trace 306 and the resistive tongue 304.
The resistance of the resistive material is typically about 50 ohms per square. Ohms per square is a unit of measure known and used in the art to describe the surface resistivity of a material, typically measured with a four point probe. With the four point probe, the resistance is measured by passing a fixed current though two points and measuring the voltage at the other two points. By controlling the input current, the surface resistance equals the voltage across the pair of test points, such that the units of distance drop out.
The size of the circular area, or xe2x80x9cdotxe2x80x9d, determines the low frequency limit of the termination. Dot diameters up to 15 times the trace width will typically perform to the upper frequency limit of a microstrip. Minimum dot diameters are typically at least three times the trace width. As an example, Table 1 shows the appropriate 20 dB and 15 dB low end frequencies of various dot sizes on a 10 mil alumina substrate.
The typical return loss performance of a dot termination at high frequencies, such as up to about 110 GHz, is better than 25 dB.
In accordance with the present invention, a dot termination composed of a circular thin film resistive material connects a transmission line to a ground plane in a manner to provide a broadband high frequency performance that also goes to DC. The dot termination can use traces provided around the perimeter of the dot resistive material with vias connecting the traces to ground to provide multiple DC paths to ground. Each trace is formed with a metal portion connecting each ground via to a resistive trace portion which connects to the resistive dot material. A resistive tongue trace connects the dot material to a metal trace forming a transmission line providing a signal to the dot termination. The use of multiple DC ground paths allows the DC resistance to be approximately 50 xcexa9 without destroying the high frequency performance.
In accordance with the present invention, the dot termination can be used in a shunt configuration with an optical modulator to provide voltage biasing for the optical modulator. To maximize the biasing voltage, a DC blocking capacitor can be placed between the dot termination and ground. Biasing current can be applied at the connection of the dot termination and the optical modulator. Preferably to enhance performance, biasing current is applied between the dot termination and the blocking capacitor.